A. Field of the Invention
The present invention relates to ultrasonic wire bonding machines of the type used to weld interconnecting wires to miniature electronic devices such as integrated circuits and magnetic read/write heads used in disk drive memories. More particularly, the invention relates to a method and apparatus for arc-forming a fusion ball at the end of a bonding wire in a shielded enclosure which substantially attenuates electromagnetic interference signals emitted from the arc discharge that might damage sensitive circuitry of a workpiece near the discharge.
B. Description of Background Art
Integrated circuits are fabricated from thin slices of a semi-conducting material such as silicon, germanium, or gallium arsenide. The slices are cut into small squares or rectangles referred to as chips or dice, ranging in size from squares about 100 mil. (0.100 inch) on a side to several hundred mils on a side. Transistors, diodes, resisters and interconnecting circuit paths are formed on the chip or die by diffusing impurities into selected regions of the chip, and by depositing various conducting paths and insulating layers onto the chip.
After a semi-conductor chip or die has been fabricated as described above, it must be attached to a base or carrier forming part of a package or container to protect the delicate die from damage. Prior to packaging, conductive pads providing input and output ports to the die must be electrically interconnected to more robust leads or terminals which extend outward through a container or package used to enclose the die. These interconnections are customarily made using fine aluminum or gold wires which are ultrasonically, thermosonically or thermo-compression welded to the pads and leads by a bonding tool that applies ultrasonic energy, a combination of heat and ultrasonic energy, or heat and pressure, respectively, to a bonding site. Since the connection pads of a microcircuit are extremely tiny and closely spaced, great precision is required in positioning the tip of a bonding tool relative to the microcircuit.
A preferred method of ultrasonically or thermosonically bonding interconnecting wires to the die pads and external leads of an integrated circuit is referred to as ball and stitch bonding. The method employs a fine gold or aluminum wire which is fed towards a work piece through a longitudinally disposed coaxial bore of a generally cylindrically-shaped bonding tool. The bonding tool has a frusto-conically-shaped lower or outer end portion having a lower face provided with a chamfered wire exit opening which communicates with the bore through the tool. The upper end of the tool is vibrated by an ultrasonic transducer, so that when the end of a wire is pressed against a metallic bonding site such as a conductive pad on a semiconductor die, or a lead, and vibrated by the transducer, an intermetallic bond or weld is made between the wire end and the bonding site.
Typical ultrasonic bonding tools used in the semiconductor industry have bore lengths of an inch or so, and diameters of 0.0015 inch to 0.002 inch (38 xcexcm to 50 xcexcm) and use gold bonding wires having a diameter of 0.001 inch to 0.00125 inch (25 xcexcm to 30 xcexcm). Such bores as well as the tools themselves are usually referred to in the industry as capillaries.
In the usual method of bonding interconnecting wires to an integrated circuit using a capillary bonding tool, the end of a bonding wire protruding from the chamfered exit opening of the tool is first melted or fused and allowed to cool, forming a solidified ball having a diameter ranging from about 1.5 to 3 times the diameter of the wire. This process is called xe2x80x9cflaming offxe2x80x9d and is accomplished by use of a fine diameter hydrogen flame, or by an electrical arc discharge. The latter technique is referred to as xe2x80x9cElectronic Flame Off,xe2x80x9d or xe2x80x9cEFO.xe2x80x9d
After a ball has been formed at the end of a bonding wire by either of the two flame-off methods described above, the capillary tool is lowered towards a workpiece and the ball is pressed against a conductive pad on the upper surface of a semiconductor die or other such miniature electronic component. Ultrasonic energy is then applied to the upper end of the tool, causing the lower end of the tool to vibrate frictionally in contact with the pad. Frictional heating produced by the vibration causes the lower surface of the ball to partially remelt and weld to the conductive pad, thus forming a ball bond. In thermosonic bonding, externally applied heat is used in conjunction with ultrasonic energy to aid in melting the interfacial surfaces between the ball and pad, thus facilitating formation of a ball bond.
Upon completion of a ball bond between a wire and a pad, the bonding tool is moved laterally and upwardly away from the ball bond to a location overlying a desired bonding site on a package lead, with the bonding wire paying out in an arcuately curved length from the ball bond. The bonding tool is again lowered to press the lower side of the wire against the lead, and ultrasonic energy applied to the tool to make a xe2x80x9cstitchxe2x80x9d bond. The bonding tool is then raised, along with clamping jaws located above the tool which grip the wire, severing the wire at its thinnest cross section, near the outer lateral end of the stitch bond. To complete the ball and stitch bonding cycle, a ball is formed on the severed end of the bonding wire protruding from the bore of the bonding tool, preparing the wire to make another ball bond.
A typical microcircuit has dozens or even hundreds of leads; therefore, it can be appreciated that ball formation in the circuits of the microcircuit must be repeated many times during the wire bonding process. For that reason, most capillary ultrasonic wire bonding machines incorporate computer controlled motor drive mechanisms which repetitively and cyclically position the bonding tool tip and wire end near a hydrogen torch flame or electrical flame-off electrode to form a ball. Automated ball-forming mechanisms are utilized in bonding machines which employ manually operated micropositioners to position the tool tip relative to a workpiece, such as those described in U.S. Pat. Nos. 3,474,685 and 5,871,136 issued to the present inventor, as well as in automatic bonding machines such as the West Bond Model 2400B, described in U.S. Pat. No. 4,125,798.
Since the amount of energy delivered by an electrical arc discharge is more readily and precisely controllable than that delivered by a hydrogen flame, the size of the fusion ball may be more closely controlled by using the EFO. Accordingly, the EFO method has been increasingly used by the semiconductor industry. However, formation of bonding wire balls by an arc discharge has heretofore been problematic, because it is believed that electromagnetic interference (EMI) energy emitted during the discharge can damage sensitive electronic components. This effect is particularly noticeable in such components as read/write heads for magnetic disk memories, which are so constructed as to produce induced voltages from relatively small changes in magnetic flux. Thus, the relatively large transient electromagnetic fields produced by an arc discharge near such workpieces can induce destructively large voltages and/or currents in the workpiece. The present invention was conceived of to address the aforementioned problem, by providing a method and apparatus for arc-forming a fusion ball at the end of a bonding wire, while attenuating potentially damaging EMI emissions from the arc discharge.
An object of the present invention is to provide a method for forming a fusion ball at the end of a bonding wire of an ultrasonic bonding machine by means of an electrical arc discharge, while minimizing electromagnetic interference (EMI) emitted from the discharge towards a workpiece.
Another object of the invention is to provide an apparatus for arc-forming a fusion ball at the end of a bonding wire which incorporates means for attenuating EMI produced by the arc discharge.
Another object of the invention is to provide a method and apparatus for forming a fusion ball at the end of a bonding wire by an arc discharge which attenuates emissions of EMI from the discharge that are incident upon a workpiece.
Another object of the invention is to provide a method for arc-forming a fusion ball at the end of a bonding wire in which the wire end is inserted into a spark chamber which is electromagnetically shielded to attenuate EMI emissions from a ball-forming arc within the chamber.
Another object of the invention is to provide a method for forming a fusion ball at the end of a bonding wire in which an ultrasonic bonding tool containing the bonding wire is displaced laterally away from the line of action of the tool and moved towards an electromagnetically shielded spark chamber to thereby insert a wire end into the chamber and form a fusion ball thereon by an arc discharge within the chamber.
Another object of the invention is to provide an electromagnetically shielded spark chamber for forming a fusion ball at the end of a bonding wire while attenuating EMI emissions from a ball-forming arc discharge to the wire end.
Another object of the invention is to provide a shielded spark chamber for arc-forming a fusion ball at the end of a bonding wire which includes an electrostatic shield comprised of a conductive shell forming a closed circuit.
Another object of the invention is to provide a shielded spark chamber for arc-forming a fusion ball at the end of a bonding wire which includes a magnetic shield comprised of a ferromagnetic shell.
Another object of the invention is to provide an apparatus for arc-forming a fusion ball at the end of a bonding wire which includes a spark chamber containing an electromagnetically shielded cup having protruding into the interior space thereof an insulated high voltage conductor terminating in an exposed conductive end, means for translating an ultrasonic bonding tool and wire end protruding therefrom to a location proximate the cup, means for moving the wire end into the cup proximate the high voltage conductor end, and means for supplying a high voltage pulse to the conductor end of sufficient voltage to form an arc to the wire end and of sufficient energy to melt the wire end and form thereon a fusion ball.
Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims.
It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, we do not intend that the scope of our exclusive rights and privileges in the invention be limited to details of the embodiments described. We do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims.
Briefly stated, the present invention comprehends an improved apparatus and method for forming a fusion ball at the end of a wire to be used for making ultrasonic ball bonds on a workpiece, in which electromagnetic interference (EMI) emissions potentially damaging to sensitive electronic circuitry of the workpiece are substantially attenuated.
According to the present invention, an ultrasonic bonding tool is moved so as to position the end of a fine gold wire protruding from the capillary bore of the bonding tool within an open upper end of a spark chamber which is shrouded on the sides and bottom thereof by electrically conductive and/or ferromagnetic shields. The shields are effective in attenuating electromagnetic interference (EMI) radiating from an arc discharge used to form a fusion ball on the wire end within the spark chamber, thus reducing the amplitude of transient electromagnetic waves incident upon a workpiece on a work plane located beneath the ball-forming arc. A preferred embodiment of a spark chamber in the apparatus according to the present invention comprises a copper block having in an upper face thereof a blind cylindrical bore which receives a cylindrical cup made of a ferromagnetic material such as PERMALLOY that has a high relative magnetic permeability. The spark chamber includes an insulated high voltage conductor which protrudes through aligned apertures through the block and cup, the high voltage conductor terminating in an electrode having an exposed conductive tip centered within the cup.
According to the method of the present invention, a capillary ultrasonic bonding tool, which has a length of bonding wire protruding outwardly from a capillary bore through the tool, is moved laterally rearward from a vertical line of action between the tool and a workpiece. The tool tip is then moved downwardly to insert the wire end into an opening provided in a shielded spark chamber located above the plane of the workpiece, where an electrical spark arcing from a high voltage electrode to the severed wire end within the chamber delivers a controlled amount of energy to the wire end sufficient to melt the wire end and form thereat a fusion ball of a predetermined diameter, when the melted wire end cools and solidifies. The tool tip is then moved upwardly, withdrawing the wire end from the spark chamber, and moved laterally forward to the line of action of the tool tip, preparatory to moving the tool tip downwards along the line of action to bond the ball end of the wire to a bond site on the workpiece.